Skip privileged pings against localhost on Windows.

Compare to this similar test in Nmap's scan_engine.cc:
  if (scantype != CONNECT_SCAN && Targets[0]->ifType() == devt_loopback) {
    log_write(LOG_STDOUT, "Skipping %s against %s because Windows does not support scanning your own machine (localhost) this way.\n", scantype2str(scantype), Targets[0]->NameIP());
    return;
  }

rndbit made a report--unfortunately we con't fix it, but we can make the
error message clearer.
http://seclists.org/nmap-dev/2013/q1/187